Method of prestressing units with serration



April 5, 1955 F. LEONHARDT METHOSQQF PRESTRESSING UNITS WITH SERRATIONFiled April 17, 1952 INVENTOR Fritz Leonhard? I ATTORNEY United StatesPatent METHOD OF PRESTRESSING UNITS WITH SERRATION Fritz Leonhardt,Stuttgart, Germany Application April 17, 1952, Serial No. 282,776

6 Claims. (Cl. 25-154) 1. This invention relates generally to methodsand means for applying prestressing units to concrete masses such asgirders or the like for the purpose of prestressing such concretemasses.

2. Channels provided in a concrete mass to receive a prestressing unitare frequently curved or contain a number of curves or bends.

3. In particular, this invention relates to a method and means ofproviding the curved portions of the channels provided in the concretefor the prestressing units with surfaces which will make it possible toreadily insert and prestress the prestressing units, reducing thefriction which normally occurs at these curves or bends.

In prestressed concrete wherein the tension is applied subsequent tohardening the prestressing members (wires, strands, or bars) are usuallyenclosed in sheet metal sleeves in order to have them longitudinallymovable in the concrete when the tension is applied. The channels aresometimes made with the aid of rubber hoses which are pulled out aftersetting of the concrete so as to allow the insertion of the prestressingunits into the resulting tubular cavities or channels. The prestressingunits are mostly curved and, due to the stresses produced by bending,they are forced during the prestressing operation against the walls ofthe cavities or channels so that the sliding movement of theprestressing unit is prevented by friction.

The use of sheet metal sleeves will result in relatively low frictionvalue. However, it has the disadvantage of producing a low bondingstrength of the cement slurry, during its subsequent serration with thesmooth metal sheets, so that the desirable narrow spacing for limitedprestressing cannot be secured. Although in channels made with the aidof rubber hoses, the bond of the cement slurry with the surroundingconcrete is satisfactory and, as a result, the serration becomes strong,this is accompanied by the development of increased friction so thatonly moderate curvatures can be used.

The invention avoids these disadvantages by using conventional rubberhoses for forming the sliding channels. However, it provides at each ofthe curves, rings of hard steel or bent tubular sections pulled over therubber hoses. The hoses are then inflated so that their outside diameterbecomes greater than the inside diameter of the steel rings or tubularsections. Moreover, the rubber hose is shaped, or tightened with a wirespiral placed underneath the exterior rubber layer, in such a mannerthat its surface becomes uneven after being inflated and, as a result, aprofiled surface is likewise produced in the hardened concrete. Byevacuating the air or liquid from the rubber hose after hardening of theconcrete, its diameter decreases to such an extent that it can be pulledout of the hardened concrete. The steel rings remain in the concrete andform suitable sliding surfaces. In curves having small radii it ispossible to provide double tubular sheet metal pieces having an inter-Patented Apr. 5, 1955 mediate layer of lubricant in order to lessen thefriction to an even greater extent. The longitudinal profile given theconcrete improves the bonding action so that with adequately shaped andsufficiently thin prestressing steel sections the bonding action becomesso eflicient that a small spacing is obtained and, as a result, limitedprestressing with a relatively high concrete tension, is made possible.

In the drawings Figure l is a view in section illustrating oneembodiment of the invention using rings to reduce friction in a concretechannel.

Figure 2 is a view in section of another embodiment of the inventionusing a tubular metal section to reduce this friction.

Referring to the drawings, in Figure 1, the hose 1 is shown as inflatedwith rings 2 constricting the hose at the curved portion of the concretechannel. As shown in both Figures 1 and 2, the flexible member or rubberhose 1 is shaped or tightened with a wire spiral 4 placed beneath theexterior rubber layer in such manner that when the hose is inflated asshown, it is constricted at certain points and its surface becomesuneven and a complementary profiled surface is produced in the hardenedconcrete.

Figure 2 shows the use of a bent tubular section 3 in place of the rings2 of Figure 1.

It is to be understood that various modifications can be made withoutdeparting from the spirit of the invention and that the invention is notlimited to the specific embodiments herein described and claimed.

I claim:

1. In a method of prestressing concrete which comprises forming achannel for prestressing units in said concrete by placing therein priorto hardening a flexible tube provided with rings or tubular sectionsthereon, said rings or tubular sections being of somewhat less internaldiameter than the external diameter of the flexible tube andsubsequently removing said flexible tube to leave said rings or sectionsin said channel with their inner faces extending out of the inner faceof the concrete.

2. A method as set out in claim 1 in which the channel has a curvedportion and the rings or tubular sections are provided in said curvedportion.

3. A method as set out in claim 1 in which the flexible tube is expandedor inflated before removal to provide enlarged channel portions adjacenteach ring or tubular section.

4. A method as set out in claim 1 in which the flexible tube is providedwith a wire spiral and the tube is subsequently expanded or inflated toprovide enlarged channel portions adjacent the convolutions of thespiral.

5. A method as set out in claim 1 in which prestressing units are placedin said channel after removal of said flexible tube.

6. A method as set out in claim 1 in which prestressing units are placedin said channel after removal of said flexible tube and cement slurry isintroduced into said channel and surrounding said prestressing units.

References Cited in the file of this patent UNITED STATES PATENTS1,650,307 Temple Nov. 22 1927 1,949,650 Lindas Mar. 6, 1934 2,413,990Muntz Jan. 7, 1947 2,648,439 Miller Aug. 11, 1953 FOREIGN PATENTS239,375 Great Britain Sept. 10, 1925

